Electrodeposition of osmium

ABSTRACT

An electrolytic bath is provided from which osmium is electroplated. The bath is comprised of an aqueous alkaline solution having dissolved therein an octavalent osmium compound and an alkali metal salt of sulfamic acid.

United States Patent Inventor Lawrence Greenspan New York, N.Y.

Appl. No. 843,174

Filed July 18, 1969 Patented Nov. 23, i971 Assignee Engelhard Minerals 8: Chemicals Corporation Newark, NJ.

ELECTRODEPOSITION 0F OSMIUM 14 Claims, No Drawings 44, I09, 123; ll7/l30 E; 106/1 [56] References Cited UNITED STATES PATENTS 2,451,340 10/1948 Jernstedt 204/47 X OTHER REFERENCES Frederick A. Lowenheim, Modern Electroplating, pp. 3 I0- 325, (1968). T5670 E46C.2

Primary ExaminerG. L. Kaplan Attorneys-Samuel Kahn and Miriam W. Lei? ELECTRODEPOSITION OF OSMIUM BACKGROUND OF THE INVENTION This invention relates to the electrodeposition of osmium and particularly to a novel osmium electroplating bath and a method of electrodepositing osmium using such bath.

Osmium, a member of the platinum group metals, is a dense hard metal. It has a low electron emissivity, and it has recently been found that various high temperature materials such as titanium, tantalum, or tungsten coated with osmium are highly useful as low-emission grid wires in cathode-ray tubes. For this use and others, it would be highly desirable to have a simple commercially feasible method for forming suitable coatings. However, very little work has been done on fonning osmium coatings. This can undoubtedly be attributed in a large measure to a combination of the scarcity of the metal, its high cost, and technological difficulties encountered in prior art techniques.

Generally, attempts to electrodeposit osmium have met with little success. The deposits have been unsatisfactory in that they were not adherent, not of a practical thickness and/or the baths deteriorated rapidly. For example, it is known to electrodeposit osmium from an aqueous neutral or slightly acid solution of an alkali metal chlorosmate to obtain a deposit of osmium on platinum. This type of bath is of little value since the plating solution is not very stable and the efficiency of the bath falls off considerably in use. It has been reported, for example, that in attempting to increase the thickness of the osmium deposit, black crystals are formed, the solution gradually darkens turning from yellow to redbrown, and the yields that can be obtained with the aged solution are very low. An alkaline bath containing NaOH, Na,co,, Na HPO and OsO. has been tried but, similarly, the deposits were not of practical thickness nor were they adherent.

It is a principal object of this invention to provide a method wherein osmium may be electrodeposited. A further object is to provide a method of electrodepositing osmium on various base metals. Another object is to provide adherent osmium electroplate of practical thickness which is characterized by a smooth lustrous appearance. Still another object is to provide a novel osmium electroplating bath which may be readily controlled and operated and which has long life.

These and other objects are attained by the use of the bath and method of this invention.

INVENTION In accordance with this invention an electrolytic bath is provided from which osmium can be electroplated as a thick adherent deposit. The bath is comprised of an aqueous alkaline solution having dissolved therein an octavalent osmium compound and an alkali metal salt of sulfamic acid.

The preparation of the electrolytic bath is effected by dissolving an octavalent osmium compound, preferably osmium tetroxide, in an aqueous alkaline solution containing an alkali metal sulfamate ,and heating the solution to effect a color change from red-brown to straw yellow. The color change is believed to be due to the formation of an osmium sulfamate complex. This heating step is an essential feature in the preparation of the bath since at best only flash coatings can be obtained with the red-brown solution. The yellow solution which forms on heating, when used as the electrolyte bath for the deposition of osmium, gives thick adherent deposit and the bath has long life.

The osmium metal content of the electrolyte bath is at least 0.5 gram per liter (g./l.) and is advantageously about 0.5 to 20 g./l. Preferably the osmium content is about 1 to l g./l. As noted above, the bath is formulated from an octavalent compound, preferably Os0 The sulfamate is a salt of sulfamic acid, e.g. sodium or potassium sulfamate. The amount of sulfamate used is adjusted, depending on the osmium content selected. As indicated above, it is believed that the color change which takes place on heating the solution containing octavalent osmium and a sulfamate is due to the formation of an osmium sulfamate complex, and sufficient sulfamate should be present to complex essentially completely the osmium present in the bath. Preferably the sulfamate should be present in excess over the osmium content. Generally, the sulfamate content (calculated on the basis of sulfamic acid) is at least about five times (by weight) the osmium metal content. Preferably the sulfamate content is considerably greater, e.g. about 10 times the osmium content. In operation of the plating process, the sulfamate content can build up to almost saturation without harmful effects.

It is desirable for the bath to contain a buffering agent. An alkali metal phosphate is preferred for this purpose, however, other inorganic buffering salts can be used, such as alkali metal carbonates or borates.

The solution is strongly alkaline and advantageously the pH is in the neighborhood of 13 to 14. The alkalinity is provided by an alkali metal hydroxide, e.g. KOH or NaOH.

By way of example, a particularly suitable bath consists of an aqueous solution of an alkali metal hydroxide, an alkali metal phosphate, an alkali metal sulfamate, and osmium tetroxide, and has a pH of l3 to l4.

The bath is operated at a temperature in the range of about 50 to 90 C., preferably about 60 to C., and at a current density of about 2 to 40 amperes per square foot (ASF). preferably 4 to 20 ASF. Agitation of the bath may be from moderate to strong.

For the insoluble anode material, platinum is preferred, although other conventional anode materials such as stainless steel may be used. The bath container may be of any material that will withstand attack by the alkaline solution under the operating conditions.

Under the foregoing conditions good coherent smooth lustrous deposits of 50 microinches and higher can be formed. Indeed smooth adherent deposits of up to 100 to 200 microinches are formed from such solutions and the baths exhibited good stability, e.g. retaining their yellow color during the entire plating operation. The current efficiency during such plating process is in the neighborhood of 40 to percent.

During the electrodeposition of osmiu'm, it is preferred to periodically add a replenishing solution to the bath. The replenishing solution suitably contains about 2 to 10 g./l. of osmium and a sulfamate in the amount of at least l0 times the amount of osmium. It is treated before use, e.g. by bringing to a boil, to effect the color change from red-brown to yellow before use.

Using the foregoing process, osmium may be plated directly on various conductive materials, such as platinum, rhodium, molybdenum, tungsten, titanium, tantalum, gold, silver, nickel, copper, etc. and alloys thereof. In plating some materials such as molybdenum or tungsten, a strike of gold is beneficial. Osmium may also be overplated with various metals, if desired, such as any of the precious metals, e.g. platinum, palladium, rhodium, ruthenium, iridium, gold or silver.

The following specific example, showing the preparation of a preferred plating bath and its use for the electrodeposition of osmium, will further illustrate this invention.

EXAMPLE a. Preparation of Bath To 160 cc. of H 0 containing 15 grams of sulfamic acid was added sufficient KOH to neutralize the acid. Then 3 grams NaOH, 3 grams Na HPO. and 2 grams OsO was added. The volume of the resulting solution was adjusted to 200 cc. by the addition of H 0 and contained 7.5 g./l. of osmium. The redbrown solution was heated to a boil for 5 minutes and became yellow in color. The solution had a pH of about 14.

b. Preparation of Replenishing Solution To 80 cc. of water was added 5 grams of KOH, 2 grams of sulfamic acid and 1 gram of OsO,. The volume was adjusted to cc. and the solution was heated to boiling for about 5 minutes.

c.Plating The plating was conducted by separating the anolyte from the catholyte by means of a porous ceramic cup. A Pt anode was employed and the agitation was from moderate to strong. As plating substrates, gold-plated brass and gold-plated copper strips were used. The bath temperature was maintained at 7080 C. At approximately one-half hour intervals replenishing solution was added to maintain the current efficiency and osmium content of the bath. Current efficiencies of 40-90 percent were obtained at current densities of 4-8 ASF. Good bright coherent osmium deposits of about 50 microinches were obtained and also good coherent smooth osmium deposits of about 100 microinches were obtained in this manner.

After 8 weeks of operation, utilizing the bath for about 4 hours a day for '5 days a week, the bath showed no signs of deterioration.

What is claimed is:

1. An electrolytic bath for depositing smooth adherent osmium coatings which comprises an aqueous alkaline solution having dissolved therein an octavalent osmium compound in an amount sufficient to provide at least 0.5grams per liter of osmium and an alkali metal salt of sulfamic acid in an amount at least sufficient to complex essentially completely the osmium compound in the bath.

2. An electrolytic bath of claim 1 wherein the osmium metal content is about 0.5 to 20 grams per liter and the sulfamate salt content is at least sufficient to complex essentially completely the osmium compound in the bath.

3. An electrolytic bath of claim 2 wherein the pH is about l3 to 14.

4. An electrolytic bath of claim 3 comprising osmium tetroxide, an alkali metal sulfamate, an alkali metal hydroxide and an inorganic buffering salts selected from the group consisting of alkali metal phosphates, carbonates and borates.

5. An electrolytic bath of claim 4 wherein the inorganic buf-= fering salt is an alkali metal phosphate.

6. A process for preparing an electrolytic bath for depositing smooth adherent osmium coatings which comprises admixing an octavalent osmium compound and an alkali metal salt of sulfamic acid in an aqueous alkaline solution and heating said aqueous alkaline solution to efiect a color change.

7. A process of claim 6 wherein the octavalent osmium compound is osmium tetroxide.

8. A process of claim 6 wherein the aqueous alkaline solution has a pH ofabout l3 to 14.

9. An electrolytic bath prepared according to the process of claim 6. t

10. A process for the electrodeposition of osmium which comprises admixing an octavalent osmium compound and an alkali metal salt of sulfamic acid in an aqueous alkaline solution, heating said aqueous alkaline solution to effect a color change from red-brown to yellow, and thereafter utilizing said yellow solution as an electrolytic bath to deposit osmium cathodically.

11. A process of claim 10 wherein the osmium is deposited cathodically at a temperature of about 50 to C. and at a current density of about 2 to 40 amperes per square foot.

12. A process of claim 10 wherein the octavalent osmium compound is osmium tetroxide.

13. A process of claim 10 wherein the aqueous alkaline solution contains an alkali metal hydroxide and an inorganic bufi'ering salt.

14. A process of claim 13 wherein the inorganic buffering salt is an alkali metal phosphate. 

2. An electrolytic bath of claim 1 wherein the osmium metal content is about 0.5 to 20 grams per liter and the sulfamate salt content is at least sufficient to complex essentially completely the osmium compound in the bath.
 3. An electrolytic bath of claim 2 wherein the pH is about 13 to
 14. 4. An electrolytic bath of claim 3 comprising osmium tetroxide, an alkali metal sulfamate, an alkali metal hydroxide and an inorganic buffering salt selected from the group consisting of alkali metal phosphates, carbonates and borates.
 5. An electrolytic bath of claim 4 wherein the inorganic buffering salt is an alkali metal phosphate.
 6. A process for preparing an electrolytic bath for depositing smooth adherent osmium coatings which comprises admixing an octavalent osmium compound and an alkali metal salt of sulfamic acid in an aqueous alkaline solution and heating said aqueous alkaline solution to effect a color change.
 7. A process of claim 6 wherein the octavalent osmium compound is osmium tetroxide.
 8. A process of claim 6 wherein the aqueous alkaline solution has a pH of about 13 to
 14. 9. An electrolytic bath prepared according to the process of claim
 6. 10. A process for the electrodeposition of osmium which comprises admixing an octavalent osmium compound and an alkali metal salt of sulfamic acid in an aqueous alkaline solution, heating said aqueous alkaline solution to effect a color change from red-brown to yellow, and thereafter utilizing said yellow solution as an electrolytic bath to deposit osmium cathodicalLy.
 11. A process of claim 10 wherein the osmium is deposited cathodically at a temperature of about 50* to 90* C. and at a current density of about 2 to 40 amperes per square foot.
 12. A process of claim 10 wherein the octavalent osmium compound is osmium tetroxide.
 13. A process of claim 10 wherein the aqueous alkaline solution contains an alkali metal hydroxide and an inorganic buffering salt.
 14. A process of claim 13 wherein the inorganic buffering salt is an alkali metal phosphate. 